An Efficient Protocol for the Synthesis of Unsymmetrical Pyrazines. Total Synthesis of Dihydrocephalostatin 11

Total Synthesis of Ritterazine B

The first total synthesis of the cytotoxic alkaloid ritterazine B is reported. The synthesis features a unified approach to both steroid subunits, employing a titanium-mediated propargylation reaction to achieve divergence from a common precursor. Other key steps include gold-catalyzed cycloisomerizations that install both spiroketals and late stage C-H oxidation to incorporate the C7' alcohol.

A Facile Synthesis of Symmetrical Dimeric Steroid-pyrazines

The reaction of steroidal 2α-bromo-3-ketones ( e.g. 2α-bromocholestan-3-one 2) with ammonia followed by hydrolysis and air-oxidation affords the easily separable mixture of the trans and cis dimeric steroid-pyrazines (3 and 4, respectively).

Regioselective addition of Grignard reagents to N-acylpyrazinium salts: synthesis of substituted 1,2-dihydropyrazines and Δ5-2-oxopiperazines

The regioselective addition of Grignard reagents to mono- and disubstituted N-acylpyrazinium salts affording substituted 1,2-dihydropyrazines in modest to excellent yields (45–100%) is described. Under acidic conditions, these 1,2-dihydropyrazines can be converted to substituted Δ⁵-2-oxopiperazines providing a simple and efficient approach towards their preparation.

A convergent total synthesis of the potent cephalostatin/ritterazine hybrid -25-epi ritterostatin GN1N

The convergent synthesis of 25-epi ritterostatin GN1N is described for the first time, starting from hecogenin acetate (HA). Stereoselective dihydroxylation employing the chiral ligand (DHQ)2PHAL was used as the key step to introduce the C25 epi-stereocenter on the north 1 segment. The title compound was obtained through a coupling reaction between the C3-keto-azide (cstat North 1) and North G.

Synthesis of 14',15'-dehydro-ritterazine Y via reductive and oxidative functionalizations of hecogenin acetate

An analog of ritterazine Y was synthesized from hecogenin acetate in 23 steps via functional group manipulations of hecogenin acetate. Preparation of the north G and south Y units and the late stage Guo-Fuchs asymmetric coupling of the both units afforded the ritterazine Y analog.

Cephalostatins and ritterazines

This review article is a tribute to the numerous chemists whose relentless effort for the last quarter of a century resulted in the isolation, identification, and finally the chemical synthesis of a family of bis-steroidal pyrazine alkaloids of marine origin. In the task of defeating cancer, the search for bioactive substances among the naturally occurring compounds is, without any doubt, a preferential approach. The remarkable contribution of Petitt, Fusetani, and their coworkers allowed to discover this family of marine alkaloids that emerge as potential therapeutic anticancer agents, although there is still a long way to go. The challenging and dangerous task of collecting living organisms from deep-waters was followed by a laborious isolation, elucidation of the complicated structures and biological tests. The outcome of this paramount effort was the identification of 45 compounds that stand, to date, as some of the most potent anticancer agents. The intriguing structures of the isolated alkaloids drew the attention of synthetic chemists, valiant enough to undertake the challenging task of synthesizing some of the most active members of the family. Fuchs, Heathcock, Winterfeldt, Suarez, Shair, and their associates pioneered in the establishment of feasible synthetic routes for the preparation of some of the naturally occurring compounds and a large number of synthetic analogs, allowing to establish SAR criteria that have guided the design of new synthetic analogs. Numerous analogs have been prepared to investigate the mechanism of action of bis-steroidal pyrazines, e.g. cephalostatin analogs bearing a strained spiroketal moiety. However, the mechanism of action and the biological target of these compounds remain far from being understood. Therefore, the rational design of simpler, yet highly active analogs seems at the current stage elusive. It is still 1 to clear why these compounds need to be dimeric to show high biological activity. Furthermore, it is not known whether the central pyrazine ring is simply a linker or has some additional function. This could be tested by examining the biological activity of steroidal dimers with other linkers, e.g. with a benzene ring. Such analogs have been actually prepared but without functional groups necessary for biological activity. The clinical trials of cephalostatins have got stuck due to a shortage of material. There is an urgent need to provide highly active, yet not too complex analogs, which could be available in substantial amounts for advanced pharmacological studies.

Enantioselective synthesis of (+)-cephalostatin 1

This Article describes an enantioselective synthesis of cephalostatin 1. Key steps of this synthesis are a unique methyl group selective allylic oxidation, directed C-H hydroxylation of a sterol at C12, Au(I)-catalyzed 5-endo-dig cyclization, and a kinetic spiroketalization.

Review of cytotoxic cephalostatins and ritterazines: isolation and synthesis

The cephalostatins and ritterazines comprise a family of structurally related natural products reported by Professors G. R. Pettit and N. Fusetani from 1988 -1998. Isolated from the invertebrate marine chordates Cephalodiscus gilchristi and Ritterella tokioka, the cephalostatins and ritterazines exhibit potent cytotoxicity toward the murine P388 lymphocytic leukemia cell line. In fact, cephalostatin 1 ( 1, ED 50 0.1-0.001 pM) proved to be one of the most powerful cancer cell growth inhibitors ever tested by the U.S. National Cancer Institute. The ritterazines and cephalostatins share many common structural features in which two highly oxygenated steroidal units with side chains forming either 5/5 or 5/6 spiroketals are fused via a pyrazine core. Professor P. L. Fuchs and colleagues reported the total syntheses of 1, cephalostatins 7 ( 7), and 12 ( 12), ritterazines K ( 30) and M ( 32), and cytotoxic analogues. The synthesis of 1, described in 1998, required 65 synthetic operations to complete.

Synthesis and structure-activity studies of schweinfurthin B analogs: Evidence for the importance of a D-ring hydrogen bond donor in expression of differential cytotoxicity

The synthesis and biological evaluation of several enantioenriched schweinfurthin B analogs were undertaken to develop structure-activity relationships and guide design of probes for their putative molecular target. The desired stilbenes contain a common left-half hexahydroxanthene ring system and an aromatic right-half with varied substituents. The synthesis involves penultimate Horner-Wadsworth-Emmons coupling of one of several right-half phosphonates with the aldehyde comprising the left-half of 3-deoxyschweinfurthin B. Preparation of the requisite phosphonates, and the respective stilbenes, as well as the cytotoxicity profiles of these new compounds in the National Cancer Institute's 60 cell-line anticancer screen is described. Several of these analogs displayed cytotoxicity patterns well-correlated with the natural product and differences in activity of approximately 10(3) across the various cell lines. Together, these assay results indicate the importance of at least one free phenol group on the aromatic D-ring of this system for differential cytotoxicity.

Polyphosphoric acid trimethylsilyl ester promoted intramolecular acylation of an olefin by a carboxylic acid: convenient construction of C-18-functionalized delta14-hecogenin acetate

[reaction: see text]. Polyphosphoric acid trimethylsilyl ester (PPSE)-promoted intramolecular Friedel-Crafts reactions on a nonaromatic carboxylic acid system have been investigated. Studies led to the synthesis of C-18 functionalized steroidal compounds 5 and 9a-d with strict retention of the spiroketals. Isomerization of spiroketal 9e was studied.

Cephalostatin analogues--synthesis and biological activity

Starting off in the early 90's the field of cephalostatin analogues has continually expanded over the last 10 years. First syntheses prepared symmetric analogues like 14b (119) and 26 (65), which were subsequently desymmetrized to provide analogues like beta-hydroxy ketone 31 (19). Importantly the straightforward approach provided already compounds with mu-molar potency and the same pattern of activity as cephalostatin 1 (1) (see Chapter 2.1). Chemically more demanding, two new methods for the directed synthesis of (bissteroidal) pyrazines were devised and subsequently applied to a wide variety of differently functionalized coupling partners. These new methods allowed for the synthesis of various analogues (Chapter 2.2.; and, last but not least, for the totals synthesis of several cephalostatin natural products; Chapter 1.). Functionalization and derivatization of the 12-position was performed (Chapter 2.1 and 3) and synthetic approaches to establish the D-ring double bond were successfully investigated (Chapter 3). [figure: see text] Dealing synthetically with the spiroketal moiety, novel oxidative opening procedures on monomeric delta 14, 15-steroids were devised as well as intensive studies regarding spiroketal synthesis and spiroketal rearrangements were conducted (Chapter 3.2. and 4.). Last but not least direct chemical modification of ritterazines and cephalostatins were studied, which provided a limited number of ritterazine analogues (Chapter 4.). All these synthetic activities towards analogues are summarized in Fig. 18. During this period of time the growing number of cephalostatins and ritterazines on the one hand and of analogues on the other hand provided several SAR trends, which can guide future analogue synthesis. The combined SAR findings are displayed in Fig. 19. So far it is apparent that: Additional methoxylations or hydroxylations in the steroidal A ring core structure (1-position) are slightly decreasing activity (compare cephalostatin 1 1 to cephalostatins 18, 19, 10, and 11). Not investigated by preparation of analogues. Additional hydroxylations in the B-ring (7- and 9-position) do not have a strong effect. They appear to decrease slightly the activity in the case of 9-position (compare cephalostatin 1 1 to cephalostatin 4) and are neutral in the case of the 7-position (compare ritterazines J and K). Analogue synthesis confirmed this: 7-ring-hydroxylation has little impact on activity, e.g. 109a (Table 6). C'-ring aryl compounds with a 12,17 connected spiroketal area are much less active (cephalostatins 5 and 6), meaning South 6 moiety reduces activity [figure: see text] Confirmed by analogue synthesis, e.g. 190a and 190b (Table 9). Regarding 12-functionalization it is apparent, that all cephalostatins/ritterazines possess either a free hydroxy or a keto function at this position (exemption: cephalostatins 5 and 6--very low activity). However, it is not apparent whether a 12,12'-diol or a 12-keto-12'-ol is favored. In the cephalostatin series the most potent compounds possess a 12-keto-12'-ol function, while in the ritterazine series the direct comparison of ritterazine B and ritterazine H clearly favors the 12,12'-diol setting. Synthesis of simple analogues like 31 showed a "cephalostatin trend" for favoring the 12-keto, 12'-alcohol functionalization. Synthesis of a cephalostatin 1-12'-alcohol 1a supported that trend (2 fold drop in activity). Synthesis of acylated ritterazine B derivatives proved that free hydroxy groups in 12-position are necessary for high activity. At least one 14,15-double bond is part of all highly active cephalostatins/ritterazines. All ritterazines lacking this feature display only low potency (but most of them possess the unfavorable North A moiety or have unfavorable combinations of moieties; vide infra). However, the 14,15-double bond may be necessary "only" for stereochemical reasons creating a specific "curvature" of the molecule by "bending" the D-ring down (for an in depth discussion on this topic: see Chapter 3). In line with this are the observations that 14,15-alpha-epoxides do substantially decrease activity (cephalostatins 14 and 15) while a 14,15-beta-epoxide does not decrease activity (cephalostatin 4). Also in line with the "curvature theory" is the fact that ritterazine B (14-beta-hydrogen) is even more potent than ritterazine G (14,15-double bond). Therefore it is not clear if--at least one--14,15-double bond is essential for high activity. The synthesis and biological evaluation of completely 14-beta-saturated analogues (like 14'-beta-hydrogen ritterazine B) could answer this question. Synthesis of the partially saturated analogues 14' alpha-cephalostatin 1 1c and 7-deoxy-14' alpha-ritterazine B 2a showed that the stronger the divergence of conformation implied by the saturation is, the higher is the loss of activity, thus underlining the "curvature hypothesis". Synthesis showed, that analogues possessing the 14,15-double bond(s) are substantially better soluble, e.g. 26. Furthermore, the D-Ring area turned out to be sensitive for modifications, since substantially differing analogues, like 162, 163, and 164 were completely inactive. At least one 17-hydroxy group is part of all highly active cephalostatins/ritterazines. Loss of one out of two 17-hydroxy groups does not decrease activity (compare ritterazine K and L) but of the second 17-hydroxy groups (along with the 7-hydroxy group) as seen in the ritterazine series (compare ritterazines A/T and B/Y) leads to a significant decrease in activity. Increased activity of 17-ether analogues 178 and 179 points into the same direction All highly active cephalostatins and ritterazines are substantially asymmetric. Cephalostatins and ritterazines that are symmetric--either consisting of two polar units (cephalostatin 12 and ritterazine K) or two unpolar units (ritterazine N and ritterazine R)--or almost symmetric (cephalostatin 13 and ritterazine J, L, M, O, S) show substantially diminished potency. However, one has to keep in mind, that even some of the symmetrical compounds (e.g. ritterazine K--96 nM in the NCI panel) still show strong cytostatic properties. Same trend was identified with simple analogues, e.g. compare 26 to 31. In addition to the basic requirement of overall substantial asymmetry for high activity there appears to be the necessity for a "polarity match" between both steroidal units (33)--as one has to be substantially more polar (high hydroxylation grade) than the other. (e.g. cephalostatin 1 (1): North 1--high hydroxylation grade--and South 1--low hydroxylation grade; or: ritterazine B (2): South 7--medium hydroxylation grade--and North G--very low hydroxylation grade). Not directly confirmed by Analogue Synthesis--some "polarity matched analogues" did not show appropriate activity, e.g. 198 and 197. 4 core moieties are privileged, meaning all highly active ritterazines/cephalostatins (see table 1) are constructed out of them. Namely these are North 1, South 1, South 7 and North G. Numerous analogues were prepared to probe questions regarding the mechanism of action of the cephalostatins, e.g. close cephalostatin analogues like 197 and 198 (70) with increased energy content in the spiroketal. However, so far the mechanism and mode of action of the cephalostatins remains unknown. In the absence of any structural information of the biological target(s), the understanding about the structural necessities for high cytostatic activity is still limited and thus the rational design of more simple, yet highly active analogues seems at the current stage elusive. Additionally, there are many open questions, e.g. how the "monomeric" OSW-1 (3) relates to the "dimeric" cephalostatins. It remains the hope that forthcoming studies will bring light into this so far nebulous area--enabling chemists in the long run to provide highly active analogues in substantial amounts for advanced pharmacological studies. In conclusion one can state that the first decade after the extraordinarily complex cephalostatin 1 (1) entered the scene was necessary for the chemists to explore novel ways towards cephalostatins and cephalostatin analogues. They have provided methods to prepare basically every thinkable cephalostatin analogue, have delivered simple analogues (< 10 steps) with substantial activity and shaped first SAR trends in the class of cephalostatins. Now the time has come for chemists to harvest the fruits of their long and enduring synthetic ventures by aiming towards highly active, yet still not too complex analogues, which could be available in substantial amounts for advanced pharmacological studies. And for pharmacologists to explore the therapeutic potential of the cephalostatins along with elucidation of the unknown mechanism. Clearly, there is much more to expect of the cephalostatins in the coming years.

An efficient synthesis of the C-23 deoxy, 17 alpha-hydroxy South 1 hemisphere and its cephalostatin 1 analog

[reaction: see text] Methods for functionalization of the D ring of diene 1 were investigated. This study led to efficient syntheses of 3 and the subsequent 23'-deoxy,17'alpha-hydroxy cephalostatin 1 analog (4). The bioactivity data of 2 and 4 are in the low nanomolar range for a 10-cell-line minipanel.

Synthesis of pyrazine-phosphonates and -phosphine oxides from 2H-azirines or oximes

[reaction: see text] Tetrasubstituted pyrazines containing two phosphonate groups 2 in positions 2 and 5 and trisubstituted pyrazines containing a phosphonate 5 or a phosphine oxide group 7 in position 2 are obtained by thermal treatment of 2H-azirine-2-phosphonates 1 and -phosphine oxides 6. These pyrazines can also be prepared from beta-ketoxime tosylates 9 and 10 or from oxime derived from phosphine oxide 11.

Dyotropic rearrangement facilitated proximal functionalization and oxidative removal of angular methyl groups: efficient syntheses of 23'-deoxy cephalostatin 1 analogues

Oxidative functionalization (or removal) of a steroidal C18 methyl group is possible using a previously unknown dyotropic rearrangement of a seven-membered fused C-ring lactone to a 6-ring spiro lactone. Spiroketal equilibration led to the 23-deoxy South analogue of cephalostatin 1 (1) in only 12 steps (23% overall yield) from hecogenin acetate 4, and to strained diene South 1 analogue 30 in 11 steps (28% overall). Total synthesis of 23'-deoxy cephalostatin 1 (3) was accomplished in 16 operations from 4 (9% overall; average 86% yield per operation), and that of 16',17'-dehydro-23'-deoxy cephalostatin 1 (36) in 15 operations from 4 (8% overall; av 84%/op).

The first total synthesis of (corrected) ritterazine M

Hecogenin acetate was converted to ritterazine M in 16 operations with an average yield per opearation of 87%. The overall linear yield was 12%. This confirmed 1 as the corrected structure for ritterazine M by total synthesis.

Outer-ring stereochemical modulation of cytotoxicity in cephalostatins

[structure: see text] 20- and 25'-epimers of cephalostatin 7, prepared by directed unsymmetrical pyrazine synthesis, address outer-ring topographical and stability questions and intimate an oxacarbenium ion rationale for the role in bioactivity of the spiroketal (E/F, E'/F') rings of this class of antitumor agents.

On topography and functionality in the B-D rings of cephalostatin cytotoxins

Analogues 12'beta-hydroxycephalostatin 1 (9), 7'-deoxyritterazine G (10), and 14-epi-7'-deoxyritterazine B (11) were prepared via our protocol for unsymmetrical pyrazine synthesis. Cytotoxicity against human tumors was also determined for the first time for ritterazines, with femtomolar potency and a high correlation to cephalostatins observed. The SAR of these and related compounds provide insight into the importance of topography and certain chemical functionality in the B-D and B'-D' rings of cephalostatin type antineoplastics.

On the relationship of OSW-1 to the cephalostatins

Antineoplastic bis-steroidal (cephalostatin-type) analogues of the saponin OSW-1 were produced from a dihydroaglycone of OSW-1. The key aglycone 6H was obtained from 5alpha-androstan-3beta-ol-17-one in 8 steps (38% yield). The SAR of the aglycones, intermediates, and hybrid analogues provide insights regarding the proposed common role of C22-oxocarbenium ions in the bioactivity of both OSW-1 and cephalostatins.

Methyloxime-substituted aminopyrrolidine: a new surrogate for 7-basic group of quinolone

Novel fluoroquinolones containing oxime functionalized aminopyrrolidines have been synthesized. They were found to possess potent antibacterial activities against both Gram-negative and Gram-positive organisms, including methicillin resistant Staphylococcus aureus (MRSA). Among these compounds, LB20277 (compound 12) showed the most favorable in vivo efficacy and pharmacokinetic profile in animals. Based on these promising results, LB20277 was selected as a candidate for further evaluation.